Spectrophotometric Quantitation of Chlorophyll $a$, Chlorophyll $b$ & Carotenoids via Arnon’s Equations
Chlorophylls are the principal light-harvesting pigments of oxygenic photosynthesis. Both Chlorophyll $a$ ($text{Chl } a$) and Chlorophyll $b$ ($text{Chl } b$) share a cyclic tetrapyrrole porphyrin (chlorin) ring chelating a central $text{Mg}^{2+}$ ion, esterified to a hydrophobic $C_{20}$ phytol tail. They differ structurally by only a single substituent at carbon-3 of ring II:
• $text{Chl } a$ possesses a methyl group ($-text{CH}_3$).
• $text{Chl } b$ possesses an electron-withdrawing formyl group ($-text{CHO}$).
This minor electronegative modification alters electronic resonance within the conjugated macrocycle, shifting the red absorption maximum of $text{Chl } b$ to shorter wavelengths ($645text{ nm}$) relative to $text{Chl } a$ ($663text{ nm}$ in $80%$ acetone). Because both pigments absorb at each other’s wavelength maxima, their individual concentrations cannot be read directly. Instead, Daniel I. Arnon (1949) formulated a set of simultaneous linear equations by determining the specific extinction coefficients ($alpha$) of purified pigments and inverting the absorption matrix:
$$A_{663} = alpha_{a,663} cdot C_a + alpha_{b,663} cdot C_b = 82.04 cdot C_a + 9.27 cdot C_b$$
$$A_{645} = alpha_{a,645} cdot C_a + alpha_{b,645} cdot C_b = 16.75 cdot C_a + 45.60 cdot C_b$$
Solving this system of linear equations yields Arnon’s classical equations for pigment concentrations in $text{mg/L}$ ($mutext{g/mL}$):
$$text{Chl } a (text{mg/L}) = 12.7 cdot A_{663} – 2.69 cdot A_{645}$$
$$text{Chl } b (text{mg/L}) = 22.9 cdot A_{645} – 4.68 cdot A_{663}$$
$$text{Total Chl} (text{mg/L}) = 20.2 cdot A_{645} + 8.02 cdot A_{663}$$
$$text{Total Carotenoids} (text{mg/L}) = frac{1000 cdot A_{470} – 1.82 cdot text{Chl } a – 85.02 cdot text{Chl } b}{198}$$
Ecophysiological Significance of the $text{Chl } a/text{Chl } b$ Ratio: $text{Chl } a$ is present in both core reaction center complexes (PSII P680 and PSI P700) and peripheral light-harvesting complexes (LHC), whereas $text{Chl } b$ is located exclusively in peripheral antenna trimers (LHCII and LHCI). Sun-adapted leaves synthesize less antenna complex per reaction center, exhibiting high $text{Chl } a/text{Chl } b$ ratios ($3.0–3.8$). Conversely, shade-adapted leaves expand their peripheral antenna to maximize photon capture under canopy shade, lowering the ratio to $2.0–2.4$.
- Plant Material: Healthy foliage from sun-grown spinach (*Spinacia oleracea*) and shade-grown money plant (*Epipremnum aureum*).
- Extraction Solvent: $80% (v/v)$ Acetone in deionized water, buffered with $2.5text{ mM Sodium Phosphate}$ ($text{pH } 7.8$) to prevent pheophytinization (acid-catalyzed loss of central $text{Mg}^{2+}$).
- Porcelain Mortar & Pestle: Pre-chilled at $-20^circtext{C}$ with acid-washed quartz sand (grinding aid) and a pinch of calcium carbonate ($text{CaCO}_3$) to neutralize vacuolar organic acids.
- Instrumentation: UV-Vis spectrophotometer with $1.0text{ cm}$ quartz/glass cuvettes, benchtop clinical centrifuge, analytical balance ($pm 0.1text{ mg}$).
Accurately weigh $0.200text{ g}$ of fresh leaf tissue, excluding midribs and major veins. Transfer to chilled mortar, add $10text{ mg } text{CaCO}_3$, a pinch of quartz sand, and $3.0text{ mL}$ of ice-cold $80%$ acetone. Grind thoroughly under dim green/subdued light until the tissue pulp is completely bleached white and homogenous.
Quantitatively transfer the green extract through Whatman No. 1 filter paper into a $25.0text{ mL}$ volumetric flask. Wash mortar and residue with small aliquots ($2text{ mL}$) of $80%$ acetone until the washing liquid is completely colorless. Bring to final mark with $80%$ acetone and invert to mix.
Transfer extract to optical glass or quartz cuvette. Zero spectrophotometer against $80%$ acetone blank. Measure and record optical densities at exactly $663text{ nm}$, $645text{ nm}$, $470text{ nm}$, and $710text{ nm}$ (residual scattering/turbidity check). Ensure $A_{710} < 0.010$; subtract $A_{710}$ from all readings.
Calculate concentrations in $text{mg/L}$. Convert to tissue yield ($text{mg pigment / g fresh weight}$) using formula: $text{Pigment (mg/g FW)} = frac{Ctext{ (mg/L)} times Vtext{ (L)}}{text{Fresh Weight (g)}} = frac{C times 0.025text{ L}}{0.200text{ g}} = C times 0.125$.
| Leaf Sample | A₆₆₃ (Chl a) | A₆₄₅ (Chl b) | A₄₇₀ (Carotenoid) | Chl a (mg/g FW) | Chl b (mg/g FW) | Total Chl | Chl a/b Ratio |
|---|---|---|---|---|---|---|---|
| Sun Leaf (Spinach) | 0.924 | 0.342 | 0.582 | 1.352 mg/g | 0.439 mg/g | 1.791 mg/g | 3.08 (High) |
| Shade Leaf (Money Plant) | 0.648 | 0.395 | 0.412 | 0.896 mg/g | 0.502 mg/g | 1.398 mg/g | 1.78 (Low) |
Sample Calculations for Sun Leaf (Spinach):
$$text{Chl } a = 12.7(0.924) – 2.69(0.342) = 11.735 – 0.920 = mathbf{10.815text{ mg/L}}$$
$$text{Chl } b = 22.9(0.342) – 4.68(0.924) = 7.832 – 4.324 = mathbf{3.508text{ mg/L}}$$
$$text{Total Chl} = 10.815 + 3.508 = mathbf{14.323text{ mg/L}}$$
Per gram fresh tissue ($25text{ mL}$ extract from $0.200text{ g}$ tissue):
$$text{Chl } a = 10.815 times 0.125 = mathbf{1.352text{ mg/g FW}}$$
$$text{Chl } b = 3.508 times 0.125 = mathbf{0.439text{ mg/g FW}}$$
$$text{Chl } a / text{Chl } btext{ Ratio} = frac{10.815}{3.508} = mathbf{3.08}$$
Total Carotenoid content:
$$text{Carotenoids} = frac{1000(0.582) – 1.82(10.815) – 85.02(3.508)}{198} = frac{582 – 19.68 – 298.25}{198} = frac{264.07}{198} = mathbf{1.334text{ mg/L}} implies mathbf{0.167text{ mg/g FW}}$$
Ecological Deduction: The $text{Chl } a/text{Chl } b$ ratio of $3.08$ in spinach reflects sun acclimation with a smaller antenna trimer pool, while the ratio of $1.78$ in money plant reflects shade adaptation with abundant LHCII complexes to harvest scattered blue-green wavelengths.
